Methyl methacrylate (MMA) as an important organic chemical product is the most primary polymerization monomer for acrylic resin plastic and has a very wide application. In China, the current process for producing methyl methacrylate is conventional acetone-cyanogen-alcohol process (ACH process). Though the ACH process is a well-established process, it has disadvantages such as high toxicity, virulent chemicals and strong acids used are harmful to environment and human beings. Accordingly, new alternative process routes have been developed, wherein a process route for preparing MMA by using C4 derivatives as raw material is mostly competitive. Comparing with other methods, the method for producing MMA by using C4 derivatives has the characteristics of low cost, less pollution, good economy benefit or the like. The process route for producing MMA by using C4 derivatives is as follows: isobutylene (tert-butanol)→methylacrolein→methacrylic acid→methyl methacrylate, or isobutylene (tert-butanol)→methylacrolein→methyl methacrylate. Methylacrolein is produced from a mixing gas of isobutylene, oxygen gas, nitrogen gas, and steam under the effect of a catalyst. The reaction product should be purified before subjecting to a one-step oxidation and esterification reaction of methylacrolein.
U.S. Pat. No. 3,957,880 provides a method for absorbing methylacrolein with ethanol, and then performing extraction and rectification using water as an extracting agent. However, water and methylacrolein tend to form an azeotrope, which cannot satisfy the demand of the second step, that is, the oxidation and esterification reaction of methylacrolein. U.S. Pat. No. 2,514,966 provides a method for absorbing methylacrolein with water. However, because the solubility of methylacrolein in water is relatively low, a great deal of water is required to absorb methylacrolein. Therefore, this method will cause water resource waste. Additionally, the obtained product also tends to form an azeotrope which is difficult to be further purified. U.S. Pat. No. 5,969,178 reports a four-column combination process of water washing, methanol drying, methanol absorption and rectification recovery which makes use of a great deal of methanol. It will cause largely waste to use methanol as an absorbent because methanol has a high volatility. Furthermore, methanol is harmful to human beings and tends to cause environmental pollution.
Therefore, it is demanded to find a recyclable and environmental friendly absorbent which can not only absorb methylacrolein effectively, but also be separated from methylacrolein easily. Ionic liquid is a green absorbent exactly satisfying the above demands.
Ionic liquid is a substance entirely consisted of a specific cation and an anion, which is in liquid state at or near room temperature. Comparing with a traditional liquid substance, the ionic liquid has particular physicochemical properties which make it more applicable to be an adsorbing solvent for methylacrolein than methanol or water:
(1) The ionic liquid has a larger solubility for the component to be separated (methylacrolein) in the mixing gas.
(2) Some types of the ionic liquids have small solubilities for other gas components except methylacrolein in the mixing gas, that is, they have higher adsorption selectivity.
(3) The ionic liquid is very low or almost zero in terms of vapor pressure, therefore, the loss caused by volatilization during adsorption and regeneration is tiny.
(4) The ionic liquid has excellent chemical stability, and furthermore, it is nontoxic and nonflammable.